Various types of humidity sensing elements, or so-called humidity elements, have been used as the tranducers of hygrometers for quantitatively sensing the water vapor content of gaseous atmospheres. Paper, or horsehair, sensing elements which respond by relatively slow changes in length dimension to changes in atmospheric moisture content have been used for many years. More sophisticated humidity sensors such as the Dunmore cell have used layers of hygroscopic chemicals such as lithium chloride as variable resistors between the electrodes of the sensors, the electrical resistance of the lithium chloride being a function of the amount of moisture absorbed from the surrounding atmosphere and measurable by electrical instrumentation. A moisture sensing element disclosed in U.S. Pat. No. 3,748,625 has a pair of electrodes spaced apart by a crystal lattice which permits molecules of the atmosphere being monitored to randomly drift in and out of the crystal interstices due to vapor pressure changes, and the volumetric resistance of the sensor changes as a function of the percent of water vapor present in the molecules of atmosphere within the interstitial spaces.
The paper or horsehair sensing elements are slow to react to moisture changes, and their reactions must be mechanically measured with the attendant problems of stickslip friction, damage possibilities, adjustment requirements, and mechanical wear problems, and do not provide the accuracy of humidity measurement which is desired in many applications. The Dunmore cell type sensors are delicate to the extent that they can be decalibrated by a fingerprint, and in that their hygroscopic nature gathers moisture from the atmosphere which may create a high humidity zone around the sensor with resultant inaccuracies in measurements. The sensor of U.S. Pat. No. 3,748,625 requires long and involved processes and results in a sensor which would appear to require special housing for physical protection.
In contrast, the present invention provides a relative humidity sensing element that may be energized or excited by low voltage microscopic currents from solid state electronic instrumentation, does not depend on mechanical movements, is physically sturdy and requires no special physical protection, is not affected by fingerprints or reasonably dirty environments, is non-hygroscopic so that moisture only permeates the element and is not attracted by it nor collected in it, has a response time on the order of one second, and is manufactured by a method comprised by a novel combination of familiar and non-exotic manufacturing methods.